U.S. patent application number 13/009531 was filed with the patent office on 2011-07-28 for system and method for providing closed loop infusion formulation delivery.
This patent application is currently assigned to Medtronic MiniMed, Inc.. Invention is credited to Ronald J. Lebel, Michael E. Miller, Rajiv Shah, TIMOTHY J. STARKWEATHER.
Application Number | 20110184380 13/009531 |
Document ID | / |
Family ID | 46301330 |
Filed Date | 2011-07-28 |
United States Patent
Application |
20110184380 |
Kind Code |
A1 |
STARKWEATHER; TIMOTHY J. ;
et al. |
July 28, 2011 |
SYSTEM AND METHOD FOR PROVIDING CLOSED LOOP INFUSION FORMULATION
DELIVERY
Abstract
A system and method for providing closed loop infusion
formulation delivery which accurately calculates a delivery amount
based on a sensed biological state by adjusting an algorithm's
programmable control parameters. The algorithm calculates a
delivery amount having proportional, derivative, and basal rate
components. The control parameters may be adjusted in real time to
compensate for changes in a sensed biological state that may result
from daily events. Safety limits on the delivery amount may be
included in the algorithm. The algorithm may be executed by a
computing element within a process controller for controlling
closed loop infusion formulation delivery. The biological state is
sensed by a sensing device which provides a signal to the
controller. The controller calculates an infusion formulation
delivery amount based on the signal and sends commands to an
infusion formulation delivery device which delivers an amount of
infusion formulation determined by the commands.
Inventors: |
STARKWEATHER; TIMOTHY J.;
(Simi Valley, CA) ; Lebel; Ronald J.; (Sherman
Oaks, CA) ; Shah; Rajiv; (Rancho Palos Verdes,
CA) ; Miller; Michael E.; (Los Angeles, CA) |
Assignee: |
Medtronic MiniMed, Inc.
|
Family ID: |
46301330 |
Appl. No.: |
13/009531 |
Filed: |
January 19, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10850637 |
May 20, 2004 |
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13009531 |
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10033173 |
Dec 26, 2001 |
6740072 |
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10850637 |
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60318062 |
Sep 7, 2001 |
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60335664 |
Oct 23, 2001 |
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Current U.S.
Class: |
604/504 ;
604/66 |
Current CPC
Class: |
A61M 2230/201 20130101;
G16H 50/30 20180101; A61M 5/1723 20130101; A61M 2005/14208
20130101; A61M 5/172 20130101; G16H 20/17 20180101 |
Class at
Publication: |
604/504 ;
604/66 |
International
Class: |
A61M 5/168 20060101
A61M005/168 |
Claims
1.-51. (canceled)
52. A method for delivering an insulin formulation, the method
comprising: taking a plurality of samples of a blood glucose level
over a period of time; calculating a difference between the sampled
blood glucose level and a predefined blood glucose level threshold;
calculating a rate of change over time in the sampled blood glucose
level; calculating a sum comprising a first proportion of said
calculated difference and a second proportion of said calculated
rate of change; and delivering an amount of insulin formulation
based on the amount of said sum.
53. The method recited in claim 52, wherein said calculated rate of
change is based on a derivative of the sampled blood glucose as a
function of time.
54. The method recited in claim 52, wherein an amount of said first
proportion or said second proportion is selectively changeable.
55. The method recited in claim 54, wherein said second proportion
is set as a first value when said sampled blood glucose level is
increasing over time and is set as a second value when said sampled
blood glucose level is decreasing over time.
56. The method recited in claim 52, wherein said sum further
comprises a basal amount of insulin formulation.
57. The method recited in claim 56, wherein said basal amount of
insulin formulation can be set to change as the part or time of the
day changes.
58. The method recited in claim 57, further comprising: disabling
insulin delivery of an amount over that of said basal amount within
a predefined amount of time of an event.
59. The method recited in claim 58, wherein said event is related
to a meal, sleep, awakening, exercise, stress-inducing event, or
medication.
60. The method recited in claim 56: wherein each sample of the
plurality of samples is taken at a time that is separated by a time
period from a time at which a next sample or a previous sample was
taken; wherein, when the sampled blood glucose level is falling
over time, said time period is of increased length as compared to a
previous time period.
61. The method recited in claim 56, wherein the basal amount or the
second proportion is zero for the purpose of said sum when the
sampled blood glucose level is below a predefined threshold and the
sampled blood glucose level is falling over time.
62. A system for delivering an insulin formulation, the system
comprising: a data acquisition unit for taking a plurality of
samples of a blood glucose level over a period of time; a
calculator for calculating a difference between the sampled blood
glucose level and a predefined blood glucose level threshold;
wherein said calculator is further configured to calculate the rate
of change over time in the sampled blood glucose level acquired by
the data acquisition unit; wherein said calculator is further
configured to calculate a sum comprising a first proportion of said
calculated difference and a second proportion of said calculated
rate of change; and a controller for basing the amount of insulin
formulation to be delivered on the amount of said sum.
63. The system recited in claim 62, wherein said calculated rate of
change is based on a derivative of the sampled blood glucose as a
function of time.
64. The system recited in claim 62, wherein an amount of said first
proportion or said second proportion is selectively changeable.
65. The system recited in claim 64, wherein said second proportion
is set as a first value when said sampled blood glucose level is
increasing over time and is set as a second value when said sampled
blood glucose level is decreasing over time.
66. The system recited in claim 62, wherein said sum further
comprises a basal amount of insulin formulation.
67. The system recited in claim 66, wherein said basal amount of
insulin formulation can be set to change as the part or time of the
day changes.
68. The system recited in claim 67, wherein said controller is
further configured to disable insulin delivery of an amount over
that of said basal amount within a predefined amount of time of an
event.
69. The system recited in claim 68, wherein said event is related
to a meal, sleep, awakening, exercise, stress-inducing event, or
medication.
70. The system recited in claim 66: wherein each sample of the
plurality of samples is taken at a time that is separated by a time
period from a time at which a next sample or a previous sample was
taken; wherein, when the sampled blood glucose level is falling
over time, said time period is of increased length as compared to a
previous time period.
71. The system recited in claim 66, wherein the basal amount or the
second proportion is zero for the purposes of said sum when the
sampled blood glucose level is below a predefined threshold and the
sampled blood glucose level is falling over time.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Embodiments of the present invention claim priority from a
U.S. Provisional Application entitled "A System and Method for
Providing Closed Loop Infusion Formulation Delivery," Ser. No.
60/335,664, filed Oct. 23, 2001, the contents of which are
incorporated by reference herein. Also, the present application
relates to co-pending U.S. Provisional Application entitled "Safety
Limits For Closed-Loop Infusion Pump Control," Ser. No. 60/318,062,
Attorney Docket No. 047711-0264, the content of which is
incorporated by reference herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally, to infusion pump
systems for the delivery of infusion formulations, and in
particular, to a closed-loop algorithm for use in conjunction with
a process controller for controlling the delivery of an infusion
formulation to a body based in part on sensed blood glucose levels
within the body.
[0004] 2. Description of Related Art
[0005] Infusion pumps have been used for the programmed delivery of
measured doses of an infusion formulation. (An infusion formulation
is defined in the present disclosure as the substance being
delivered by the infusion pump. This substance may comprise either
a mixture of different components or it may be a single, pure
substance, including, but not limited to drugs, dyes or other
indicators, nutrient, or the like.) A typical example of such use
is the delivery of an insulin formulation to a patient.
[0006] In the case where the infusion formulation is an insulin
formulation, a sensing device may regulate the delivery of the
insulin formulation by sensing the levels of blood glucose in the
person. The delivery of the insulin formulation may be controlled
by a control device associated with the pump having as an input a
sensed blood glucose level. The control device may control
activation of the pump to deliver an appropriate amount of the
insulin formulation in accordance with the sensed blood glucose
level.
[0007] Insulin is a protein hormone normally formed within the
human pancreas. Because it regulates carbohydrate (sugar)
metabolism, insulin is required for normal metabolic function. More
specifically, insulin helps the body metabolize glucose. To avoid
medical problems such as hypoglycemia and hyperglycemia, blood
glucose levels should be maintained within a specific range. A
normal range for glucose in the human body may be between 85 and
120 milligrams/deciliter (mg/dl).
[0008] In a non-diabetic person, insulin is secreted by the
pancreas in small amounts throughout the day (basal rate of insulin
secretion). In addition, the amount of insulin secreted by the
pancreas may be modified under certain circumstances. For example,
the pancreas of a non-diabetic person normally secretes larger
amounts of insulin (bolus rate of insulin secretion) when the
person ingests a meal to prevent postprandial hyperglycemia, i.e.,
abnormally increased sugar content in the blood.
[0009] In contrast to the non-diabetic person, a diabetic person's
pancreas may not secrete the required amount of insulin. Thus, the
diabetic person has to somehow artificially introduce the insulin
into the body. One method of introducing the insulin is by the
conventional insulin formulation injection method using a syringe.
Using this method, the body's blood glucose level may be monitored
(for example, by checking a blood sample) and the amount of insulin
to be injected may be adjusted accordingly. For example, after a
meal the blood glucose level may be monitored and an appropriate
amount of insulin may be injected into the bloodstream of the
user.
[0010] In the alternative, a diabetic person may choose to use an
infusion pump such as the infusion pump described above. By using
an infusion pump, a diabetic person may be able to adjust insulin
delivery rates for the pump in accordance with the user's needs.
These needs may be determined based on prior experience and/or the
results of glucose monitoring (for example, by a sensing device in
combination with a communication device).
[0011] In addition, infusion pumps may be engineered to function as
an artificial pancreas. Such an infusion pump may deliver a
specific amount of insulin formulation at specific intervals. As
discussed above, a sensing device associated with the pump may
monitor the blood glucose level of the user and the blood glucose
level may then be used by the pump to automatically regulate the
delivery of the insulin formulation.
[0012] It is known to use as a control device a process controller
for performing automatic regulation of the infusion pump. The
process controller, for example a processor or other computing
element, controls the process such that a process variable is
maintained at a desired set point value (also referred to in the
present disclosure as the "goal"). Such process controllers
typically use a set of control parameters which have been
determined through, for example, experimentation or calculation, to
operate in an optimal manner to control the process variable.
Although not the only possible technique, these control parameters
are typically dependent on the anticipated range of differences
("error values") that result between the process variable and the
set point during actual operation of the process.
[0013] Ordinarily, infusion formulation delivery systems utilize
control systems having an input-response relationship. A system
input, such as a sensed biological state, produces a physiological
response related to the input. Typically, the input (such as a
sensed blood glucose level) is used to control some parameter
associated with the response variable (such as an insulin infusion
rate or an amount of insulin).
[0014] A process controller employed in the delivery of an insulin
formulation typically executes a closed-loop algorithm that accepts
and processes a blood glucose level input supplied to the
controller by a sensing device. The closed-loop algorithm may
adjust insulin formulation delivery as a function of, for example,
the rate of change over time of the sensed glucose level.
[0015] These closed-loop algorithms have many limitations. Some of
these limitations result from the fact that a process controller
employing a closed-loop algorithm to control the delivery of an
insulin formulation may be restricted to only adding insulin
formulation to the system. Once insulin formulation is added to the
system, normally the controller cannot retrieve it.
[0016] Additional limitations result from the fact that certain
parameters affecting glucose production may not be adequately
compensated for by these closed-loop algorithms. For example,
certain daily events may significantly affect glucose production
levels in the human body. Thus, these events may also significantly
affect the amount of insulin required to metabolize the
glucose.
[0017] Exercise, for example, has been shown to lower blood glucose
levels in the human body. Thus, exercise may result in a dip in
blood glucose levels and a corresponding decrease in the amount of
insulin formulation delivered by the body. Longer or more strenuous
exercise events may result in a greater dip in blood glucose level
than shorter and less strenuous exercise events.
[0018] Similarly, sleep and stress may affect the body's ability to
burn carbohydrates and therefore may affect glucose levels. For
example, glucose metabolism has been found to be slower in a sleep
deprived state. In addition, elevations of certain stress hormones
within the body may also result in slower glucose metabolism. Thus,
longer or shorter periods of sleep or stress may result in more or
less significant changes in glucose levels.
[0019] Furthermore, the ingestion of certain medications may affect
a user's sensitivity to insulin, i.e. a given amount of insulin may
be more or less sufficient depending on whether or not a particular
medication has been taken.
[0020] An additional event that may significantly affect the
production of glucose in the body is the ingestion of food. This
results in part from the fact that during digestion carbohydrates
are broken down into glucose that then enters the bloodstream. In
addition, the amount and type of foods ingested affect the amount
of glucose produced.
[0021] Closed-loop algorithms employed for controlling delivery of
an insulin formulation in response to sensed blood glucose levels
may not adequately compensate for the affects such daily events may
have on blood glucose levels. Thus, the diabetic person relying on
such closed-loop algorithms may be at an increased risk of
hypoglycemia and/or hyperglycemia.
SUMMARY OF THE DISCLOSURE
[0022] Therefore, it is an advantage of embodiments of the present
invention to provide a closed-loop algorithm for controlling
delivery of insulin formulation which more accurately calculates an
infusion formulation delivery rate based on a level of blood
glucose which is sampled in a body at predefined intervals.
[0023] It is a further advantage of embodiments of the present
invention to provide a closed-loop algorithm for controlling
delivery of insulin formulation which may be adjusted in real time
to more accurately determine whether a blood glucose level is
rising or falling over a predetermined interval.
[0024] It is a further advantage of embodiments of the present
invention to provide safety limits for bolus delivery that may be
compared with samples of blood glucose parameters at predefined
intervals and which enable or disable bolus delivery based on the
comparisons.
[0025] It is a further advantage of embodiments of the present
invention to provide safety limits on the amount of insulin
formulation that may be stored in an accumulator during a
predefined time interval.
[0026] It is a further advantage of embodiments of the present
invention to provide safety limits on the amount of insulin
formulation that may be delivered to a user during a predefined
time interval.
[0027] These and other advantages are accomplished according to
embodiments of a closed-loop algorithm for use in conjunction with
a process controller for delivering an infusion formulation.
Components of the closed-loop algorithm calculate a present value
of infusion formulation in a body as well as whether that value is
rising or falling overall during a predefined time interval. The
closed-loop algorithm includes an equation whose variables are
programmable in real time. The variables may be used as control
parameters which may be adjusted to adjust the algorithm to more
accurately calculate the present value of infusion formulation in
the body.
[0028] Preferred embodiments of the present invention provide a
closed-loop algorithm for use with a proportional-derivative
controller for delivering an insulin formulation which comprises an
equation for calculating a proportional component, a derivative
component, and a basal component of an amount of insulin
formulation to be delivered based on a sensed blood glucose level.
Control parameters within the closed-loop algorithm may be
programmable in real time and may be adjusted to compensate for
events which may significantly affect the blood glucose level.
[0029] Depending upon the context of use, the invention may include
various combinations of these features which function together to
provide both adjustable control parameters and safety limits on the
delivery of infusion formulation in response to a detected
biological state. Various embodiments of the invention include one
or more of these features.
[0030] These and other objects, features, and advantages of
embodiments of the invention will be apparent to those skilled in
the art from the following detailed description of embodiments of
the invention, when read with the drawings and appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 shows a block diagram of an infusion formulation
delivery system utilizing a control system having an input-response
relationship, according to preferred embodiments of the
invention;
[0032] FIG. 2 shows a flow diagram of a general process performed
by a closed-loop algorithm for adjusting infusion formulation
delivery as a function of a change in a sensed biological
state;
[0033] FIG. 3 shows the operation of a closed-loop algorithm used
by a proportional-derivative controller;
[0034] FIG. 4 shows a flow diagram 400 illustrating a process for
implementing a filter order, according to an embodiment of the
invention;
[0035] FIG. 5A shows a blood glucose response curve after a higher
filter order for the falling side of the curve has been
implemented, according to one embodiment of the present
invention;
[0036] FIG. 5B shows a magnified view of a portion of the response
curve of FIG. 5A;
[0037] FIG. 6 shows flow diagram 600 to illustrate effects of
implementing time windows, according to an embodiment of the
invention;
[0038] FIG. 7 shows a graph of a human blood glucose response for a
user who has ingested a meal, illustrating effects of implementing
a time window, according to an embodiment of the invention;
[0039] FIG. 8 shows a graph of a human blood glucose response for a
user who has ingested a meal, illustrating effects of implementing
time windows, according to an embodiment of the invention;
[0040] FIG. 9 shows flow diagram which illustrates effects of
increasing the value of x in the trend term of Equation 4 when the
trend term first indicates that the blood glucose level is falling,
according to an embodiment of the invention;
[0041] FIG. 10 shows a flow diagram illustrating effects of a
programmable trend gain on the present calculated value of the
infusion formulation, according to an embodiment of the
invention;
[0042] FIG. 11 shows a graph of a human blood glucose response for
a user who has ingested a meal, illustrating a trend up gain and a
trend down gain, according to an embodiment of the invention;
[0043] FIG. 12 shows a flow diagram illustrating effects of
disabled and enabled trend terms, according to an embodiment of the
invention;
[0044] FIG. 13 shows a graph of a human blood glucose response for
a user who has ingested a meal, illustrating effects of disabled
and enabled trend terms, according to an embodiment of the
invention;
[0045] FIG. 14 shows, a flow diagram illustrating effects of the
basal rate component, according to an embodiment of the
invention;
[0046] FIG. 15 shows a graph of a human blood glucose response for
a user who has ingested a meal, illustrating effects of the basal
rate component, according to an embodiment of the invention;
[0047] FIG. 16A shows a graph of a human blood glucose response for
a user who has ingested a meal, illustrating a process whereby a
pump stroke volume is accumulated, according to an embodiment of
the invention;
[0048] FIG. 16B shows a magnified view of a portion of the response
curve of FIG. 16A; and
[0049] FIG. 17 shows a flow diagram illustrating a verification of
the status of each bolus control parameter before a bolus delivery
is executed, according to an embodiment of the invention.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0050] In the following description of preferred embodiments,
reference is made to the accompanying drawings which form a part
hereof, and in which is shown by way of illustration specific
embodiments in which the invention may be practiced. It is to be
understood that other embodiments may be utilized and structural
changes may be made without departing from the scope of preferred
embodiments of the present invention.
Environment of Use
[0051] As discussed above, embodiments of the present invention
relate to a closed-loop algorithm for use in conjunction with a
process controller for controlling the delivery of an infusion
formulation to a body based in part on a sensed biological state
within the body.
[0052] Embodiments of the invention may be employed in various
infusion environments including, but not limited to a biological
implant'environment. In preferred embodiments, the closed-loop
algorithm is employed for use in conjunction with a delivery device
such as an infusion pump utilized in an implant environment within
a human body. However, other embodiments may be employed for use in
other biological implant or non-implant environments, including but
not limited to external infusion devices, pumps or the like.
[0053] Furthermore, in example embodiments described herein, the
closed-loop algorithm is employed for use in conjunction with an
infusion pump configured for delivery of an insulin formulation
used to regulate glucose levels in a diabetic user. However, other
embodiments may be employed in the delivery of other infusion
formulations having other pharmacological properties.
Closed-Loop Control System
[0054] A block diagram of an infusion formulation delivery system
100 utilizing a control system having an input-response
relationship according to preferred embodiments of the invention is
shown in FIG. 1. A sensor 102 generates a sensor signal 112
representative of a system parameter input 110 (such as a blood
glucose level of a human body 108), and provides the sensor signal
112 to a controller 104. The controller 104 receives the sensor
signal 112 and generates commands 114 that are communicated to the
infusion formulation delivery device 106. The infusion formulation
delivery device 106 then delivers the infusion formulation output
116 to the body 108 at a determined rate and amount in order to
control the system parameter 110.
[0055] Sensor 102 may comprise a sensor, sensor electrical
components for providing power to the sensor and generating the
sensor signal 112, a sensor communication system for carrying the
sensor signal 112 to controller 104, and a sensor housing for
enclosing the electrical components and the communication system.
Controller 104 may include one or more programmable processors,
logic circuits, or other hardware, firmware or software components
configured for implementing the control functions described herein,
a controller communication system for receiving the sensor signal
112 from the sensor 102, and a controller housing for enclosing the
controller communication system and the one or more programmable
processors, logic circuits, or other hardware, firmware or software
components. The infusion formulation delivery device 106 may
include a suitable infusion pump, infusion pump electrical
components for powering and activating the infusion pump, an
infusion pump communication system for receiving commands from the
controller 104, and an infusion pump housing for enclosing the
infusion pump, infusion pump electrical components, and infusion
pump communication system.
Closed-Loop Algorithm
[0056] FIG. 2 shows a flow diagram of a general process performed
by a closed-loop algorithm for adjusting infusion formulation
delivery as a function of, for example, the rate of change over
time of a sensed biological state. As shown in step 202, the
closed-loop algorithm checks for changes in the biological state at
timed intervals. A sensing device such as sensor 102 detects the
change in glucose level and communicates the change to a control
device such as controller 104 as an input to the closed-loop
algorithm. If no change is detected, the closed-loop algorithm
loops back to step 202, repeating this process until a change is
detected. When a change occurs at step 204, the closed-loop
algorithm determines the amount and/or rate of infusion formulation
required based on the input and various parameters that have been
programmed into the controller.
[0057] Where the infusion formulation delivery system 100 shown in
FIG. 1 includes a controller 104 used for controlling an insulin
response to a sensed blood glucose level, the closed-loop algorithm
may be of the proportional-derivative (PD) type. The use of a PD
type closed-loop algorithm is advantageous, for example, when
processing resources such as processor power and/or memory may be
limited. In alternative embodiments, a
proportional-integral-derivative (PID) type closed-loop algorithm
may be used.
[0058] PD controllers may utilize a closed-loop algorithm which
computes both a proportional component and a derivative component
of a response (output) to changes in a system parameter (input).
For example, the proportional and derivative components may be
combined to calculate an amount of insulin formulation to be
delivered in response to a present sensed blood glucose level
(system parameter input 110) within a body 108. The controller may
then issue commands 114 to, for example, output a calculated amount
of insulin formulation (output 116) to an infusion site on or
within the body 108 based on the present sensed blood glucose
level.
[0059] The magnitude of each component's contribution to the
calculated amount of insulin formulation to be delivered to the
infusion site may be expressed by a formula or equations, such as
the following equations:
U.sub.P=.alpha.(G.sub.(t)-G.sub.sp) Equation 1
and
U.sub.D=.beta.dG/dt, Equation 2
[0060] where
[0061] U.sub.P is the proportional component of the response,
[0062] U.sub.p is the derivative component of the response,
[0063] .alpha. is a proportional gain coefficient,
[0064] .beta. is a derivative gain coefficient,
[0065] G is a present blood glucose level,
[0066] G.sub.sp is a desired blood glucose level or "set point" for
the blood glucose level, and
[0067] t is the time at which the blood glucose level is
sensed.
[0068] There is a desired blood glucose level G.sub.sp for each
person which may be determined, for example, from experimentation
or from the person's historical physiological data. The closed-loop
control system may be designed to maintain the desired blood
glucose level G.sub.sp for a particular person. It may do this, in
part, by measuring the difference between the determined G.sub.sp
and a blood glucose level G sensed at time t (G.sub.(t)). This
difference is the blood glucose level error at time t that must be
corrected.
[0069] The proportional component expressed in Equation 1
determines whether the blood glucose level error is positive,
negative, or zero, (i.e., whether G.sub.(t) is, respectively,
higher, lower, or equal to G.sub.sp). Thus, G.sub.sp is subtracted
from G.sub.(t). If G.sub.(t) is higher than G.sub.sp, the
controller 104 may generate an insulin formulation delivery command
114 to drive the infusion formulation delivery device 106 to
provide insulin formulation (output 116) to the body 108. If
G.sub.(t) is lower than G.sub.sp, the controller 104 may reduce or
stop delivery of the insulin formulation to the body 108 by the
infusion formulation delivery device 106. The result of subtracting
G.sub.sp from G.sub.(t) is then multiplied by a proportional gain
coefficient .alpha.. The derivative component dG/dt expressed in
Equation 2 determines if the blood glucose level is presently
rising or falling and at what rate of change.
[0070] Thus, to determine the amount of infusion formulation to be
delivered at any point in time (I.sub.(t)), the following standard
equation may be used:
I.sub.(t)=.alpha.(G.sub.(t)-G.sub.sp)+.beta.dG/dt Equation 3
[0071] where I.sub.(t) is the amount of insulin formulation to be
delivered based on the sensed blood glucose level at time t.
Example Operation of a Closed-Loop Algorithm
[0072] Referring now to FIG. 3, the operation of a closed-loop
algorithm used by a PD controller is described. FIG. 3 illustrates
a typical human blood glucose response to the ingestion of a meal.
Shown in FIG. 3 is a graph of a blood glucose response curve 300
(on the y axis) as a function of time (on the x axis). This blood
glucose response curve 300 is representative of blood glucose
levels sensed at various sampling times as a system parameter 110
by a sensor 102, as shown in FIG. 1.
[0073] As shown in FIG. 3, after a person ingests a meal 302, there
is typically a steady rise 304 in blood glucose level over time
until the blood glucose level reaches a peak 306. It has been
observed from experimentation that peak 306 may occur approximately
90 minutes after ingestion of the meal. After peak 306 has been
reached, it has been observed that the blood glucose level then
begins to decrease 308 over time. During the decline from the first
peak 306, a second temporary rise 310 in blood glucose level has
been observed. A second peak 312 results from this temporary rise
310. This second peak 312 may occur approximately 30 to 90 minutes
after the occurrence of peak 306 and typically tends to occur 30 to
60 minutes after the occurrence of peak 306.
[0074] After peak 312 has been reached, it has been observed that
the blood glucose level then continues as before to decrease 314
over time. Although the reasons for this second, temporary rise 310
are not completely understood at the present time, it is a
consistently observable phenomenon that presents a problem for a
closed-loop algorithm.
[0075] To understand the problem, it is helpful to understand the
response of a closed-loop algorithm at the various points of the
response curve 300 shown in FIG. 3. As stated above, at point 302,
the meal is ingested. As the blood glucose level rises 304 above
the set point 316, a closed-loop algorithm may calculate both the
amount by which the present blood glucose level exceeds the set
point value (a proportional component) and may also determine that
the blood glucose level is rising at a certain rate (a derivative
component). Thus, a closed-loop algorithm may calculate a result
based on these two components which causes a command to issue from
a controller associated with the algorithm to deliver a calculated
amount of insulin at a time t on the response curve 300
corresponding to 304.
[0076] At peak 306 of the response curve 300, the blood glucose
level is neither rising nor falling, but the proportional component
calculates that it is still above the set point and therefore the
controller associated with the closed-loop algorithm may continue
to issue commands to deliver more insulin formulation, although it
may not be as large an amount as that issued at 304 on the response
curve 300.
[0077] At 308, the proportional component calculates that the blood
glucose level is still above the set point. However, now the blood
glucose level is falling, and therefore the controller associated
with the closed-loop algorithm may issue commands to deliver a
decreased amount of insulin formulation based on the calculation of
the derivative component.
[0078] At 310, the proportional component calculates that the blood
glucose level is still above the set point. The derivative
component will calculate that the blood glucose level is rising
again. At this point, the controller associated with the
closed-loop algorithm may issue a command to deliver another
significant amount of insulin based on this information although,
seen globally, the blood glucose level is decreasing overall. Thus,
because of this additional input of insulin formulation into the
system, the risks of hypoglycemia to the user are increased.
Embodiments of Closed-Loop Algorithms
[0079] Preferred embodiments of the present invention address the
limitations of a closed-loop algorithm exemplified above in
relation to FIG. 3. Preferred embodiments of closed-loop algorithms
more accurately determine the amount of insulin formulation to be
delivered based on a sensed blood glucose level by including
programmable control parameters which may be used to introduce
discontinuities in the calculation of I.sub.(t) unlike the
continuous calculations of I.sub.(t) performed by the closed-loop
algorithm described above. Embodiments of the present invention may
be more effective at maintaining a desired blood glucose level for
a particular user under circumstances where blood glucose level may
be significantly affected by events such as, but not limited to
meals, sleep, and exercise. As a result, the risk of hypoglycemia
and/or hyperglycemia in the user may be reduced.
[0080] In some embodiments of the present invention, the derivative
component of the closed-loop algorithm (dG/dt) shown in Equation 2
above is referred to as the "trend term" and may be expressed,
as:
Trend term=(G.sub.(t)-G.sub.(t-x))/x Equation 4
[0081] where x is a numerical value representing increment of
time.
[0082] In some embodiments, the value of the trend term is
calculated at predetermined intervals, for example each minute, and
is used to determine the "trend" of G, i.e., whether the value of G
is trending up or trending down during a timeframe determined by
the term (t-x). Thus, by changing the value of x, the timeframe for
sampling the trend may be lengthened or shortened. As an example,
using Equation 4, if x=10 minutes, the blood glucose level sensed
10 minutes prior in time to time t is subtracted from the blood
glucose level sensed at time t. In some embodiments, as discussed
in more detail below, the value of x may be programmable. In
alternative embodiments, linear regression or other curve-fitting
techniques may be used.
[0083] Generally, a shorter timeframe (and, thus, a smaller value
of x) is preferred for trend calculation because the shorter the
timeframe, the more responsive the infusion formulation delivery
system may be to a rising or falling blood glucose level. However,
this responsiveness must be balanced against noise susceptibility
of the sensor signal, which may increase as the timeframe gets
shorter. After the trend term is calculated, it is multiplied by
the derivative gain coefficient .beta..
[0084] The proportional gain coefficient .alpha. and derivative
gain coefficient .beta. (.beta. is also referred to in the present
disclosure as the "trend gain") may be chosen based, for example,
on experimentation. As an example, they may be chosen based on
observations of the insulin response of several, normal glucose
tolerant users. An average of the values of these responses may
then be taken. Alternatively, other statistical values besides an
average value may be used, for example a maximum or minimum value,
standard deviation value, or some other suitable value.
[0085] In some embodiments, as discussed in more detail below, both
the proportional and derivative gain coefficients may be
programmable. In addition, 3 may be programmed as one value when
the trend is going up and a different value when the trend is going
down (also referred to in the present disclosure as the "trend up"
and "trend down" gains).
[0086] It is believed that even if G.sub.(t) is equal to G.sub.sp
(in other words if the proportional component of the response is
zero), a certain minimal amount of insulin formulation should still
be delivered in order to maintain that condition. Thus, in some
embodiments, in addition to Equation 1 and Equation 2 shown above,
a basal insulin formulation delivery amount is included as a
further component of the response. This basal component (B.sub.0)
represents, in some embodiments, a minimum amount of insulin
formulation that would be delivered when G.sub.(t) is equal to or
greater than G.sub.sp (i.e., when the blood glucose level at time t
is equal to or greater than the desired blood glucose level or set
point) and without regard to the rate at which the blood glucose
level is rising or falling. In some embodiments, as discussed in
more detail below, B.sub.0 may be programmable and may be selected
from a programmable table of multiple B.sub.0 values based on
certain criteria. By selecting B.sub.0 values from this
programmable table, different values of B.sub.0 may be selected for
different parts of the day (for example, dawn). Thus, different
parts of the day may be treated differently than other parts of the
day.
[0087] Thus, to determine the amount of infusion formulation to be
delivered at any point in time (I.sub.(t)) the following equation
may be used by embodiments of the present invention:
I.sub.(t)=.alpha.(G.sub.(t)-G.sub.sp)+.beta.((G.sub.(t)-G.sub.(t-x))/x)+-
B.sub.0 Equation 5
Higher Order Filters For Down Trend
[0088] Generally, the body's blood glucose level changes slowly
compared to the rate at which the sensor 102 samples these levels.
Therefore, high frequency signal components are typically noise.
Referring again to FIG. 1, in some embodiments of the present
invention sensor 102 may further include a filter. The filter may
be used to reduce noise seen in sensor signal 112 in particular
frequency bands prior to being received by controller 104. In some
embodiments, a low pass filter such as, but not limited to, a
finite impulse response ("FIR") filter, is used for this purpose.
This filter may be adjusted to pass lower frequencies and stop
higher frequencies.
[0089] By increasing the order of the FIR filter, a sharper cutoff
in the frequency response of the low pass filter may be achieved.
In one embodiment of the present invention, the order of the filter
may be programmable and different orders of the filter may be
implemented based on whether the blood glucose level response curve
(for example, response curve 300 in FIG. 3) is rising or
falling.
[0090] FIG. 4 shows a flow diagram 400 illustrating the process for
implementing a filter order. As illustrated in flow diagram 400, in
one embodiment the derivative component of Equation 5 may be
sampled at step 402. If the derivative component of Equation 5 is a
positive value or zero, i.e., if the blood glucose level is rising
or at a peak, the filter order may be maintained as shown in step
404. If the derivative component of Equation 5 is a negative value,
i.e., if the blood glucose level is falling, a higher order filter
may be implemented at step 406. As a result of implementing a
higher order filter when the blood glucose level is falling, the
temporary peaks on the falling side of the response curve (such as
peak 312 in FIG. 3) may be flattened, as illustrated in FIGS. 5A
and 5B.
[0091] FIGS. 5A and 5B illustrate the effects of this embodiment of
the present invention on a response curve such as response curve
300. FIG. 5A shows a response curve 500 after the higher filter
order for the falling side has been implemented according to one
embodiment of the present invention described above. FIG. 5B shows
a magnified view of a portion of the response curve referred to in
FIG. 5A by numeral 518.
[0092] It can be seen from FIG. 5B that the second peak 512
(corresponding to second peak 312 in FIG. 3) has been flattened as
a result of the higher order filter. Thus, the derivative component
of the closed-loop algorithm may not detect as steep a rise and may
reduce the amount of insulin formulation delivered as a result of
this second peak 512. Therefore, as a result of implementing
embodiments of the invention, the risk of hypoglycemia to the user
may be reduced.
Disabling Closed-Loop Algorithm During Predefined Time Window
[0093] In another embodiment of the present invention, after a meal
has been ingested by a user, the amount of insulin formulation to
be delivered based on a sensed blood glucose level may be more
accurately determined by establishing, for example from historical
physiological data, a time window within which the temporary rise
in blood glucose level occurs in the user. Once this time window
has been established, embodiments of the present invention may
disable any further commands from issuing from the controller (for
example, commands 114 from controller 104 in FIG. 1), by, for
example, programming start and stop times for the time window that
may be used by the controller to suspend any further calculations
of I.sub.(t) during the time window.
[0094] FIG. 6 shows flow diagram 600 which illustrates the effects
of implementing time windows, as described above. As illustrated in
flow diagram 600, in one embodiment the current time t may be
sampled and compared at step 602 to the programmed start and stop
times to determine if time t is within the programmed time window.
If time t is not within the programmed time window, the issuance of
commands based on Equation 5 may be enabled at step 604. If time t
is within the programmed time window, the issuance of commands
based on Equation 5 may be disabled at step 606 until the
programmed stop time. In this way, minimal or no additional insulin
formulation may be delivered during the time window, as illustrated
by the graph shown in FIG. 7.
[0095] FIG. 7 shows a graph of a human blood glucose response 700
for a user who has ingested a meal at the point in time referred to
by numeral 702. For the purposes of illustration, it will be
assumed that it has been established from the user's historical
physiological data that the second rise occurs in the user at the
time referred to by numeral 724. Thus, in the present example, the
second peak 712 occurs approximately two hours after the meal is
ingested. Thus, the time window for disabling commands from being
issued by the controller may be set between a disable start time,
referred to by numeral 726, and a disable stop time, referred to by
numeral 728. After time 728 is reached, the controller commands may
again be enabled.
[0096] It can be seen from FIG. 7 that because the second rise 710
and resulting second peak 712 occur within the programmed time
window, the second rise does not result in any increase in
delivered insulin formulation. This discontinuity in the
calculation of I.sub.(t) may thus cause I.sub.(t) to be calculated
based only on the global downward trend of response curve 700.
Therefore, as a result of implementing one embodiment of the
invention, the temporary rise 710 does not cause any increase in
the amount of delivered insulin formulation, and the risk of
hypoglycemia to the user is reduced.
Programmable Control Parameters For Equation 5
[0097] In yet another embodiment of the present invention, the
amount of insulin formulation to be delivered based on a sensed
blood glucose level may be more accurately determined by having
control parameters in Equation 5 which are programmable. In some
embodiments, higher accuracy is achieved by including some control
parameters which may be programmable in real time, i.e., while the
closed-loop control system is in operation. Table 1 shows the
control parameters within Equation 5 that may be programmable in
different embodiments of the present invention. In some
embodiments, all the control parameters shown in Table 1 are
programmable. In one embodiment, the control parameters shown in
Table 1 may be programmed in real time. Table 1 also includes
example values for each control parameter.
TABLE-US-00001 TABLE 1 Control Parameter Value Glucose Set Point
(G.sub.sp) 100 mg/dl Basal Rate (B.sub.0) 0.5 units/hour
Proportional Gain (.alpha.) 0.01 units/hour Trend Term 2
mg/dl/minute Trend Up Gain (.beta.) 1.0 units/hour * (mg/dl/minute)
Trend Down Gain (.beta.) 3.0 units/hour * (mg/dl/minute)
[0098] Some embodiments of the present invention use the
programmable control parameters shown in Table 1 to advantageously
adjust the closed-loop algorithm to compensate for changes in the
blood glucose level that result from events such as, but not
limited to, a meal event. The temporary rise in blood glucose level
seen a period of time after the meal has been ingested is an
example of a change in blood glucose level resulting from an event.
Other events that may require compensation for changes in the blood
glucose level include, but are not limited to exercise, illness,
stress, sleep and other events which may induce metabolic changes.
Some embodiments may adjust the control parameters to compensate
for the temporary rise so that it does not result in the delivery
of a significant amount of insulin formulation. Thus, these
embodiments decrease the risks of hypoglycemia to the user.
[0099] In one embodiment, the timeframe of the trend term of
Equation 4 may be lengthened by increasing the programmable value
of x. This embodiment is illustrated by the graph shown in FIG. 8,
which shows a human blood glucose response 800 for a user who has
ingested a meal at the point in time referred to by numeral 802. A
first timeframe wherein x=10 minutes is referred to by numeral 804
and defines a 10 minute timeframe'extending back in time from time
t. It can be seen that if a trend term is calculated at time t, the
trend of the blood glucose level will be calculated as rising 808
for that defined timeframe.
[0100] By increasing the value of x in the trend term, the
timeframe may be lengthened in order to decrease the responsiveness
of the infusion formulation delivery system and calculate a trend
term that is more accurate in terms of whether the blood glucose
level is globally rising or falling.
[0101] This is illustrated by a second timeframe, referred to by
numeral 806, wherein x=30 minutes and defines a 30 minute timeframe
extending back in time from time t. It can be seen that for the
majority of the period encompassed by timeframe 806 the blood
glucose level is trending downward. Thus, the overall calculation
of the trend term will result in a negative value. Thus, by
increasing the programmable value of x in order to define a longer
timeframe in which to sample the trend, a more accurate calculation
is made of I.sub.(t), thus reducing the risk of hypoglycemia to the
user.
[0102] In a further embodiment, the value of x in the trend term of
Equation 4 may be increased only for the falling side of blood
glucose response curve 800. Thus, in this embodiment, the
controller may be programmed to increase the value of x in the
trend term of Equation 4 when the trend term first indicates that
the blood glucose level is falling. In this manner, the better
responsiveness of the shorter timeframe may be maintained while the
blood glucose level is rising.
[0103] FIG. 9 shows flow diagram 900, which illustrates effects of
increasing the value of x in the trend term of Equation 4 when the
trend term first indicates that the blood glucose level is falling.
The trend may be sampled at step 902 at time t and it may be
determined whether or not the trend is falling. If the trend is not
falling, the timeframe may be maintained, as shown at step 904. If
the trend is falling, the timeframe may be increased, as shown at
step 906. In this way, the trend control parameter of the
closed-loop algorithm may be adjusted in such a way that the
temporary rise in the blood glucose level may have no, effect on
the overall, global trend of the blood glucose level over time.
[0104] Thus, the embodiment illustrated in FIG. 8 uses the
programmable trend term parameter shown in Table 1 to
advantageously adjust the closed-loop algorithm such that the
temporary rise in blood glucose level does not result in the
delivery of a significant amount of insulin formulation and thus
reduces the risks of hypoglycemia to the user.
[0105] In other embodiments of the present invention, the trend up
and trend down gain control parameters may be used to
advantageously adjust the closed-loop algorithm such that the
temporary rise in blood glucose level does not result in the
delivery of a significant amount of insulin formulation. As stated
above, the trend gain control parameter .beta. may be chosen based
on observations of the insulin response of several normal glucose
tolerant users.
[0106] It has been determined through experimentation that the risk
of hypoglycemia may be reduced by rapidly cutting off insulin
formulation delivery to the user once it is determined that the
trend is falling. In some embodiments, therefore, the trend gain
may be programmable and may have a greater value when the trend is
falling (trend down gain) and a lesser value when the trend is
rising (trend up gain).
[0107] FIG. 10 shows a flow diagram 1000 illustrating the effects
of a programmable trend gain. The trend may be sampled at step 1002
at time t and it may be determined whether or not the trend is
falling. If the trend is not falling, the trend up gain may be used
in Equation 5, as shown at step 1004. If the trend is falling, the
trend down gain may be used in Equation 5, as shown at step 1006.
In this way, the trend gain control parameter of the closed-loop
algorithm may be adjusted in such a way that the temporary rise in
the blood glucose level may have no effect on the overall, global
trend of the blood glucose level over time.
[0108] FIG. 11 illustrates why this may be advantageous in
preventing the delivery of a significant amount of insulin
formulation in response to the temporary, second rise in blood
glucose level seen after a meal. FIG. 11 shows a graph of a human
blood glucose response 1100 for a user who has ingested a meal at
the point in time referred to by numeral 1102. Also shown in FIG.
11 is a timeframe, referred to by numeral 1106, wherein x=10
minutes and defines a 10 minute timeframe extending back in time
from time t.
[0109] At time t.sub.1, the trend of the blood glucose level is
sampled and is determined to be rising 1104. Thus, the trend term
will be some positive value. As an example, the trend term may have
a value of 2 mg/dl/minute, as shown in Table 1 above. As seen in
Equation 5, this value will be multiplied by the trend gain, and
because it is positive, the trend up gain will be used. In this
example, the trend up gain is chosen as 1.0
units/hour*(mg/dl/minute), as shown in Table 1. Thus, the
derivative component of Equation 5 may be calculated as 1.0
units/hour*(mg/dl/minute)*2 mg/di/minute=2 units/hour. It can be
seen, therefore, that because, in the present example, the trend is
rising at a rate of 2 mg/dl/minute, an additional 2 units/hour of
insulin formulation is added to the proportional component and the
basal component of Equation 5.
[0110] In contrast, when the trend is falling, a larger value of
trend gain, i.e., the trend down gain, is used. Shown in FIG. 11 is
a timeframe, referred to by numeral 1110, wherein x=10 minutes and
defines a 10 minute timeframe extending back in time from time t.
At time t.sub.2 the trend of the blood glucose level is sampled and
is determined to be falling 1108. Thus, the trend term will be some
negative value. As an example, the trend term may have a value of
-2 mg/dl/minute, as shown in Table 1 above. As seen in Equation 5,
this value will be multiplied by the trend gain, and because it is
negative, the trend down gain is used. In this example, the trend
down gain is chosen as 3.0 units/hour* (mg/dl/minute), as shown in
Table 1. Thus, the derivative component of Equation 5 may be
calculated as 3.0 units/hour*(mg/dl/minute)*-2 mg/dl/minute=-6
units/hour. It can be seen, therefore, that because in the present
example the trend is falling at a rate of 2 mg/dl/minute, it is
calculated that 6 units an hour should be subtracted from the
current insulin formulation delivery rate.
[0111] In some embodiments, the trend down gain may be chosen such
that the calculation of the derivative component of Equation 5
results in a high enough negative value to completely offset the
other components of Equation 5 and, thus, to substantially cut off
further delivery of insulin formulation during the down trend, even
though the blood glucose level is currently above the set point
1116. Thus, embodiments may use a high enough value for the trend
down gain such that the temporary rise in blood glucose level may
have no effect, since the delivery of insulin formulation may be
cut off at a time t before the temporary rise occurs. Thus, the
risk of hypoglycemia to the user is reduced.
[0112] In other embodiments of the present invention, the
closed-loop algorithm advantageously disables the trend term from
contributing to I.sub.(t) under certain circumstances in order to
further reduce the risks of hypoglycemia to a user. In one
embodiment, the trend term of Equation 5 is disabled and does not
contribute to I.sub.(t) unless the trend is rising and the user's
goal blood glucose level has been reached.
[0113] This is illustrated in flow diagram 1200 shown in FIG. 12.
The blood glucose level may be sampled at step 1202 and it may be
determined whether or not the user's goal (set point) has been
reached. If the goal has not been reached, the trend term may be
disabled, as shown at step 1204. If the goal has been reached, the
trend term may be enabled, as shown at step 1206. In this way, the
closed-loop algorithm may be adjusted in such a way that a
significant amount of insulin formulation may not be delivered to
the user unless the user's blood glucose level is both rising and,
at the same time, above the user's blood glucose level set point,
thus reducing the risk of hypoglycemia.
[0114] FIG. 13 illustrates one embodiment. FIG. 13 shows a graph of
a human blood glucose response 1300 for a user who has ingested a
meal at the point in time referred to by numeral 1302. The blood
glucose level begins to rise 1304, but is still below the user's
set point value 1316. Thus, in one embodiment the derivative
component of Equation 5 is disabled and does not contribute to
I.sub.(t). When the blood glucose level reaches the set point 1316
at time t, the derivative component of Equation 5 is enabled and
begins to contribute to I.sub.(t).
[0115] Shown in FIG. 13 is a timeframe, referred to by numeral
1306, wherein x=10 minutes and defines a 10 minute timeframe
extending back in time from time t. At time t the trend of the
blood glucose level may be sampled to determine the difference
between the blood glucose level at time t and at time t-10, as
described above in relation to FIG. 11. Therefore, once the user's
blood glucose level is both rising and above the set point, the
trend term of Equation 4 (which is equivalent to the derivative
component of Equation 5) may be calculated. An additional amount of
insulin formulation determined by the calculation may then be
delivered to the user to assist in metabolizing the blood
glucose.
[0116] In other embodiments of the present invention, the
closed-loop algorithm advantageously enables and disables the basal
bate B.sub.0 component of Equation 5, which may be a programmable
control parameter (as shown in Table 1 above). In one embodiment,
the basal rate component may be enabled or disabled based in part
on whether the user's blood glucose level is above or below,
respectively, the user's set point.
[0117] As discussed above, the basal rate component B.sub.0 of
Equation 5 represents, in some embodiments, a minimum amount of
insulin formulation that would be delivered when the blood glucose
level at time t is equal to or greater than the desired blood
glucose level or set point and without regard to the rate at which
the blood glucose level is rising or falling. Embodiments
advantageously disable the basal rate component. B.sub.0 of
Equation 5 from contributing to I.sub.(t) when the blood glucose
level falls below the set point and the trend term is falling. This
may be done, for example, to substantially inhibit any further
delivery of insulin formulation when the blood glucose level has
fallen from a maximum value to a point below the set point.
[0118] FIG. 14 shows a flow diagram 1400, illustrating the effects
of the basal rate component of Equation 5. The blood glucose level
may be sampled at step 1402 and it may be determined whether or not
the user's blood glucose level is below the set point. If the blood
glucose level is not below the set point, the basal rate component
of Equation 5 may be enabled, as shown at step 1404. If the blood
glucose level is below the set point, the trend may be sampled and
it may be determined whether or not the trend is falling, as shown
at step 1406. If the trend is not falling, the basal rate component
of Equation 5 may be enabled, as shown at step 1404. If the trend
is falling, the basal rate component of Equation 5 may be disabled,
as shown at step 1408. In this way, the basal rate component of
Equation 5 would be enabled when the blood glucose level sampled at
time t is equal to or greater than the set point value regardless
of the trend direction and would be disabled when the blood glucose
level sampled at time t is less than the set point value and the
trend is falling.
[0119] FIG. 15 illustrates one embodiment. FIG. 15 shows a graph of
a human blood glucose response 1500 for a user who has ingested a
meal at the point in time referred to by numeral 1502. The blood
glucose level begins to rise 1504, but is below the user's set
point 1516. Thus, according to the one embodiment, even though the
user's blood glucose level is below the set point 1516, the basal
rate component of Equation 5 is enabled because the trend is not
falling. The blood glucose level is still rising at 1506 and is now
above the user's set point 1516. Thus, because the user's blood
glucose level is both above the set point 1516 and rising, the
basal rate component of Equation 5 is enabled. According to one
embodiment, under the conditions, described above in relation to
1504 and 1506, the basal rate component of Equation 5 is enabled
and contributes to
[0120] At 1508, the blood glucose level is falling, but is above
the user's set point 1516. Thus, even though the user's blood
glucose level is falling, it is still above the set point 1516 and,
therefore, the basal rate component of Equation 5 is enabled. At
1510, the blood glucose level is still falling and is now below the
set point. Thus, because the blood glucose level is both falling
and below the set point, the basal rate component of Equation 5 is
disabled and does not contribute to I.sub.(t). Therefore, one
embodiment substantially cuts off any insulin formulation,
including the basal rate component, when the glucose level is both
falling and below the set point. Iri this way, embodiments reduce
the risk of hypoglycemia.
[0121] Further embodiments of the present invention may include a
programmable table of basal rate values. The closed-loop algorithm
may be programmable to select particular basal rate values from the
table to be used in calculating I.sub.(t) in Equation 5, for
example, at particular times of the day. As an example, a different
basal rate value may be selected at particular time intervals
throughout the day. In one embodiment, the basal rate value may be
updated every 30 minutes. In further embodiments, other control
parameters within the closed-loop algorithm may be adjusted
differently at different times of the day.
[0122] Thus, embodiments may advantageously adjust the basal rate
based on daily events such as, but not limited to, meals, sleep,
exercise, stress inducing events, ingested medications, and the
like. In addition, embodiments enable the updating of basal rate
values based on a particular user's historical physiological data.
For example, a particular user may have a lower need for insulin at
night. For that user the closed-loop algorithm may be programmed to
use lower basal rate values at night.
Monitoring Biological States Other Than Blood Glucose Level
[0123] In further embodiments of the present invention, the amount
and/or rate of delivered insulin formulation may modified based on
inputs from sensing devices that detect other biological states in
lieu of or in addition to the sensed blood glucose level. For
example, it has been observed that a user's blood oxygen levels may
change based on whether the user is awake or sleeping. As discussed
above, sleep is an event which may significantly affect blood
glucose levels in particular users. Thus, embodiments may sense the
blood oxygen level of a user to determine if the user is asleep and
input this information to the closed-loop algorithm in order to
adjust the amount and/or delivery rate of insulin formulation based
on this information.
[0124] Similarly, it has been observed that body temperature may
significantly affect blood glucose levels. Thus, one embodiment
includes a temperature sensor which monitors body temperature and
includes this information as an input to the controller in order to
adjust the amount and/or delivery rate of insulin formulation based
on this information.
[0125] Further embodiments of the present invention may include a
sensing device for detecting whether or not a user is exercising.
In one embodiment, an accelerometer or other device suitable for
detecting motion may be used to detect motion as an indicator of
current physical activity. As discussed above, exercise may
significantly affect blood glucose levels in particular users.
Thus, information from the exercise sensing device may be input to
the controller in order to adjust the amount and/or delivery rate
of insulin formulation based on this information.
[0126] Referring again to FIG. 1, in one embodiment sensor 102 may
sense many biological states including, but not limited to, blood,
glucose level, blood oxygen level, and temperature. Sensor 102 may
further include an exercise sensing device such as an
accelerometer. In other embodiments, a separate blood glucose level
sensor, blood oxygen level, temperature sensor and exercise sensing
device may be used. Further embodiments may include sensors that
detect various combinations of these and/or other biological
states.
Reduction of Accumulated Insulin Formulation
[0127] An infusion pump for the delivery of an infusion formulation
according to some embodiments has a fixed pump stroke volume, i.e.,
there is a certain minimum value of infusion, formulation that must
be accumulated before a pump stroke is executed, referred to in the
present disclosure as a "pump stroke volume." Thus, if I.sub.(t) is
calculated on a periodic basis, for example each minute, then the
calculated amount for each minute may be some fractional part of a
pump stroke volume. These fractional parts may be stored, for
example, in a chamber or reservoir within or adjacent to the
infusion pump until an amount equal to the pump stroke volume has
been accumulated. At that time, a pump stroke may be executed and
the insulin formulation delivered.
[0128] The process where a pump stroke volume is accumulated is
illustrated with reference to FIGS. 16A and 16B. FIG. 16A shows a
graph of a human blood glucose response 1600 for a user who has
ingested a meal at the point in time referred to by numeral 1602.
FIG. 16B shows a magnified view of a portion of the response curve
referred to in FIG. 15A by numeral 1608.
[0129] The blood glucose level begins to rise 1604. At time
t.sub.1, a first value for I.sub.(t) may be calculated using
Equation 5. The amount of insulin formulation calculated as
I.sub.(t) at time t, may be some fractional part of a pump stroke
volume and may be stored in the accumulator. At time t.sub.2, a
second value for I.sub.(t) may be calculated. The amount of insulin
formulation calculated as I.sub.(t) at time t.sub.2 may also be
some fractional part of a pump stroke volume and may be added to
the first value stored in the accumulator. At time t.sub.3, a third
value for I.sub.(t) may be calculated, and so on.
[0130] At time t.sub.n, an nth value of I.sub.(t) is calculated
using Equation 5. The amount of insulin formulation calculated as
I.sub.(t) at time t.sub.n is added to the accumulator, at which
time the amount of insulin formulation in the accumulator is
equivalent to a pump stroke volume. A pump stroke may now be
executed to deliver the insulin formulation. Time t.sub.(n) may
vary based on the pump stroke volume and the intervals at which
I.sub.(t) is calculated.
[0131] As stated above, a process controller employing a
closed-loop algorithm to control the delivery of an insulin
formulation may be restricted to adding insulin formulation to the
system, i.e., a body. Once insulin formulation is added to the
system, normally the controller cannot retrieve it.
[0132] In further embodiments of the present invention, the
accumulated volume of infusion formulation may be purged from the
accumulation chamber or reservoir (also referred to in the present
disclosure as the "accumulator") when the calculation of I.sub.(t)
yields a result which shows that the blood glucose level is
falling. Thus, although once delivered the infusion formulation may
not be retrievable from the body, it may be retrieved from the
accumulator before the pump stroke is executed.
[0133] In one embodiment, at any time before a pump stroke is
executed, the controller may issue a command to purge the
accumulator. For example, once it is determined that the blood
glucose level is falling and delivery of further insulin
formulation is not desirable, the amounts of insulin formulation
that were calculated at times t.sub.1 through t.sub.n while the
blood glucose level was rising may be purged from the accumulator
once the blood glucose level begins to fall. Thus, the accumulator
may be advantageously "zeroed out." In addition, under
circumstances involving high levels of blood glucose, the
accumulator may be allowed to go negative, thus delaying the effect
of future increases in blood glucose levels.
Programmable Control Parameters For Bolus Safety Limits
[0134] In further embodiments of the present invention, a large
amount of insulin formulation (a "bolus") may be delivered by the
infusion formulation delivery device, independently of Equation 5,
when a user has a blood glucose level that is above a predefined
value and is rising at or above a predefined rate, thus possibly
indicating that a meal has been consumed. In other words, when the
predefined criteria is met, the bolus amount may be delivered
instead of a value of I(t) calculated using Equation 5.
[0135] In preferred embodiments, predefined bolus safety limits are
included as control parameters for the closed-loop algorithm. In
some embodiments, the bolus control parameters may be programmable
in real time. Table 2 shows example bolus safety limit control
parameters that may be programmable in different embodiments of the
present invention. In some embodiments, all the control parameters
shown in Table 2 are programmable. In one embodiment, the control
parameters shown in Table 2 may be programmed in real time. Table 2
also includes example values for each control parameter.
TABLE-US-00002 TABLE 2 Control Parameter Value Bolus amount Up to
25 units in increments of 0.2 units; preferably 1-8 units Time
between boluses One minute to 24 hours; preferably 30-60 minutes
Bolus threshold 50-200 mg/dl; preferably 80-160 mg/dl Bolus trend
Varies from individual to individual; typically 1-5 mg/dl/min for
humans; preferably 2-4 mg/dl/min
[0136] Preferred embodiments of the present invention use the
programmable control parameters shown in Table 2 to advantageously
provide safety limits to be used in order to reduce the possibility
of erroneously delivering a bolus by ensuring that the status of
each control parameter is verified before a bolus delivery is
executed by the infusion formulation delivery device. This is
illustrated by flow diagram 17, shown in FIG. 17.
[0137] As discussed above, the blood glucose level is sampled at
intervals, for example every minute. In some embodiments, each time
the blood glucose level is sampled, a check is performed by the
closed-loop algorithm to determine the status of the control
parameters shown in Table 2.
[0138] In one embodiment, the closed-loop algorithm first
determines if a bolus delivery feature is enabled 1702. This may be
determined, for example, by comparing a predefined "bolus amount"
control parameter value with zero. If the value is equal to zero,
bolus delivery may be disabled 1704. If the value is greater than
zero, the "time between boluses" control parameter may be checked
1706.
[0139] The "time between boluses" control parameter determines
whether or not a predefined time interval has been exceeded since
the last bolus delivery. If the time interval between bolus
deliveries has not been exceeded, bolus delivery may be disabled
1704. If the time interval between bolus deliveries has been
exceeded, the "glucose threshold" control parameter may be checked
1708.
[0140] The "glucose threshold" control parameter determines whether
or not a predefined blood glucose level has been reached. If the
predefined blood glucose level has not been reached, the bolus
delivery feature may be disabled 1704. If the predefined blood
glucose level has been reached, then the "bolus trend" control
parameter may be checked 1710.
[0141] The "bolus trend" control parameter determines whether or
not the blood glucose level is rising at a predefined rate. If the
blood glucose level is not rising at the predefined rate, then the
bolus delivery feature may be disabled 1704. If the blood glucose
level is rising at the predefined rate, then the bolus delivery
feature may be enabled 1712. Also, according to an embodiment of
the present invention, additional signal processing may be
implemented to detect a signature of a meal, which may then be used
to enable the bolus feature.
[0142] Thus, embodiments advantageously provide bolus safety limits
to reduce the possibility of erroneously delivering a bolus by
ensuring that predefined conditions for delivery of a bolus are met
by testing predefined control parameters that are programmable.
Thus, the closed-loop algorithm reduces the possibility of
delivering too much insulin formulation as a bolus and thus reduces
the risks of hypoglycemia to the user.
Programmable Control Parameters For Maximum Insulin Formulation
Delivery Amounts
[0143] In yet other embodiments of the present invention,
additional safety limits may be used to ensure that no more than a
predefined maximum amount of insulin formulation is stored in the
accumulator at each sampling interval. For example, when the
sampling interval is one minute, a limit may be set on the maximum
amount of insulin formulation that may be stored in the accumulator
each minute. This amount may be programmable.
[0144] Similarly, in yet a further embodiment, a limit may be set
on the maximum amount of insulin formulation that may be delivered
by the infusion formulation delivery device in one hour. This
amount may also be programmable.
[0145] Thus, by "clamping" the maximum amount that may be stored in
the accumulator at each sampling period and the maximum amount that
may be delivered to the body each hour, embodiments of the present
invention reduce the possibility of delivering too much insulin
formulation and thus reduce the risks of hypoglycemia to the
user.
[0146] Accordingly, a number of aspects and features of preferred
embodiments of the closed-loop algorithm described above may
provide programmable control parameters for tuning the closed-loop
algorithm to more accurately determine an amount of insulin
formulation to be delivered in response to a sensed blood glucose
level in order to reduce the risks of hypoglycemia to a user.
Additional aspects and features of preferred embodiments of the
closed-loop algorithm may provide safety limits which reduce the
risks of hypoglycemia to a user. The aspects and features described
above may be combined to provide maximum control and safety for a
user. However, the foregoing description of embodiments of the
invention has been presented for the purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise forms disclosed. Many modifications and
variations are possible in light of the above teaching.
[0147] For example, several embodiments of the closed-loop
algorithm were described above in relation to a graph of a human
blood glucose response for a user who has ingested a meal. These
examples are meant to be illustrative and not limiting. The meal
event is used as an example of an event which may lead to changes
in insulin production by the pancreas of a non-diabetic person, and
for which the tuning of the closed-loop algorithm using control
parameters may be advantageous. However, the meal event should not
be considered to be a limitation on the events which may affect
glucose levels in the human body, and thus on the events for which
adjustable control parameters for tuning the closed-loop algorithm
may be advantageous.
[0148] Thus, the programmable control parameters may be adjusted to
adjust the closed-loop algorithm to more accurately calculate the
amount of insulin formulation to be delivered during or after other
events which may affect the blood glucose response of a user. For
example, The programmable control parameters may be adjusted to
more accurately calculate the amount of insulin formulation to be
delivered during or after exercise events, medication events,
stress events, sleep events, and the like.
[0149] Having disclosed exemplary embodiments and the best mode,
modifications and variations may be made to the disclosed
embodiments while remaining within the scope of the invention as
defined by the following claims.
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